Classifications

• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05B—CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS
  • G05B19/00—Programme-control systems
  • G05B19/02—Programme-control systems electric
  • G05B19/18—Numerical control [NC], i.e. automatically operating machines, in particular machine tools, e.g. in a manufacturing environment, so as to execute positioning, movement or co-ordinated operations by means of programme data in numerical form
  • G05B19/406—Numerical control [NC], i.e. automatically operating machines, in particular machine tools, e.g. in a manufacturing environment, so as to execute positioning, movement or co-ordinated operations by means of programme data in numerical form characterised by monitoring or safety
  • G05B19/4062—Monitoring servoloop, e.g. overload of servomotor, loss of feedback or reference
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B25—HAND TOOLS; PORTABLE POWER-DRIVEN TOOLS; MANIPULATORS
  • B25J—MANIPULATORS; CHAMBERS PROVIDED WITH MANIPULATION DEVICES
  • B25J9/00—Programme-controlled manipulators
  • B25J9/16—Programme controls
  • B25J9/1674—Programme controls characterised by safety, monitoring, diagnostic
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05B—CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS
  • G05B2219/00—Program-control systems
  • G05B2219/30—Nc systems
  • G05B2219/37—Measurements
  • G05B2219/37312—Derive speed from motor current
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05B—CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS
  • G05B2219/00—Program-control systems
  • G05B2219/30—Nc systems
  • G05B2219/43—Speed, acceleration, deceleration control ADC
  • G05B2219/43201—Limit speed to allowable speed for all axis

Definitions

• the present invention relates to a method and an apparatus method for monitoring and limiting the velocity of an industrial robot having an arm including a plurality of arm parts and a robot hand , which are movable relative each other about a plurality of joints, and the movements of the joints are driven by a plu- rality of motors.
• An industrial robot includes a manipulator and a control system for automatically operating the manipulator.
• the manipulator includes at least one arm having a plurality of arm parts that are movable relative each other about a plurality of joints.
• the outer arm part supports a hand that is rotatable about one or more joints.
• the hand includes a tool attachment, also called a face- plate, adapted to be attached to an object hold by the robot, such as a tool or a work object.
• the movements of the joints are driven by motors mounted on or close to each joint. Each joint is driven by one motor.
• the speeds and accelerations of the joints are controlled by the control system of the robot that generates control signals to the motors.
• the control system comprises drive units that control the motors by converting DC current to a variable alternating current in dependence on the control signals.
• the motors are equipped with position detectors, such as encoders or resolvers, to provide position feed back signals to the control system .
• the position detectors measure the angular speed of the motors. For safety reasons, the velocity of an ob- ject hold by the robot hand has to be monitored , and if the velocity of the object exceeds a certain threshold value, an action to limit the velocity of the object is required . Since the size of the object differs between different applications and is unknown due to the fact that it is added by the customer, the velocity of the object can be difficult to supervise. Instead , it is more convenient to supervise the velocity of the faceplate of the robot hand .
• US 2009/0088898 discloses a robot controller for controlling a robot equipped with a robot hand portion including velocity calculation means for calculating moving velocities of the robot hand portion on coordinate axes of a rectangular coordinate system set for the robot controller, comparator means for compar- ing the calculated moving velocities with thresholds values for the axes of the rectangular coordinate system, and halting means for halting the robot in case any of the moving velocities is higher than the corresponding threshold value.
• the velocity calculation means calculates the velocity of the robot hand based on position data from position sensors of the motors and a kinematic model of the robot.
• a disadvantage with this method is that it is not safe since it is too slow and depends entirely on software.
• the object of the present invention is to provide a solution to the above mentioned problems.
• this object is achieve by a method as defined in claim 1 .
• the method comprises repeatably: determining a parameter related to the angular speed of the motor for each motor of a selected subgroup of motors of the robot, determining whether or not one or more predefined criteria for allowed robot arm velocity are fulfilled based on the determined motor related parameter, wherein said criteria are defined with respect to maximum allowed angular speed of the motors of said subgroup of motors, and reducing the speed of one or more of the motors of the sub- group in case any of the criteria is not fulfilled .
• the parameter is the angular speed of the motor or the frequency of the motor.
• a parameter related to the angular speed for some of the motors of the robot are used to supervise the velocity of the robot arm.
• a parameter related to the angular speed of the motor is meant the angular speed itself or a parameter that depends on the speed of the motor, such as the frequency of the motor, which is directly proportional to the angular speed of the motor.
• the velocity of the robot arm is su- pervised based on some predefined criteria, which have been defined with respect to maximum allowed angular speed of the motors and with regard to maximum allowed velocity of the robot arm . An action is taken to limit the speed of the motors if those criteria are not fulfilled .
• the invention makes it possi ble to implement both the monitoring and the limitation functionality in hardware close to the power electronics.
• the monitoring and limitation is made independent of the software used in the con- trol loops. Thereby, safety is increased . This reduces the risk for failure due to software bugs. Further, the monitoring and veloc- ity limitation becomes fast. Another advantage is that no extra sensor is needed for monitoring the velocity of the robot arm.
• the criteria for allowed robot arm velocity have been set up be- forehand based on predetermined maximum values of the arm velocity.
• the criteria have been set up based on a predetermined maximum allowed velocity of the faceplate, and/or a maximum allowed velocity of an elbow of the robot.
• the criteria include at least one predefined threshold value for the parameter for each motor of the subgroup.
• the threshold values are predetermined based on off-line calculations of worse case velocities for the robot arm, for example, for robot hand and/or the TCP of the robot. If any of the criteria is not fulfilled , the speed of one or more of the motors is limited . Preferably, the speed of the motor is reduced to zero.
• Limiting the arm velocity can be made in different ways, for example, the speed of one or more of the selected motors is limited , or the power supply to all of the motors of the robot is in- terrupted and the robot is stopped .
• the power supply to all joints are interrupted by means of deactivating the power electronics
• the number of motors in the subgroup should be less than the total number of joints of the robot.
• the subgroup includes at least two motors less than the total number of mo- tors driving the joints of the robot, to increase the performance of the robot.
• the influence on the velocity of the robot arm such as the velocity of the face plate, differs between the joints of the robot.
• the joints to be monitored , and accordingly the motors of the subgroup are selected due to their dominating influence on the velocity of the faceplate, and accordingly on the velocity of the object hold by the robot.
• each of said motors is supplied with a variable alternating power
• the method comprises measuring a variable of the variable alternating power supplied to the motor for each motor of said selected subgroup of motors, and determining said motor related parame- ter based on the measured variable.
• the variable is the voltage or the current of the supplied power.
• the method com- prises measuring a variable of the variable power supplied to each of the motors of the subgroup, determining the frequencies of the power supplied to the motors based on the measured variable, calculating the angular speeds of the motors based on the determined frequencies, and determining whether said pre- defined criteria are fulfilled or not based on the determined angular speeds and predefined threshold values for the angular speeds of the motors.
• the frequency of the power supplied to the motors can easily be determined by hardware, for example, using zero cross detector or a Phase lock Loop (PLL).
• PLL Phase lock Loop
• an apparatus as defined in claim 8 is adapted to repeatably determining a parameter related to the angular speed of the motor for each motor of a selected subgroup of motors of the robot, to determine whether or not one or more predefined criteria for allowed arm velocity are fulfilled based on the determined motor related parameters, wherein said criteria are defined with respect to maximum allowed angular speed of the motors of said su bgroup of motors, and to limit the speed of one or more of the motors of the subgroup in case any of the criteria is not fulfilled .
• the apparatus comprises a measuring system configured to measure a variable of the power supply to the motors of said selected subgroup of motors, and the apparatus is adapted to determine said motor related parameter based on the measured variable.
• the power to the motors is commonly supplied through three phases.
• the measuring system is configured to measure the variable of at least two phases of the supplied power.
• the safety unit comprises a plurality of hardware devices, such as a Phase Locked Loop (PLL) or a Zero Cross Detector, adapted to receive measurements of a variable of the variable power supplied to each of the motors of the subgroup, to determine the frequencies of the power supplied to the motors based on the measured variable, to calculate the angular speeds of the motors based on the determined frequencies, and to determine whether said predefined criteria are fulfilled or not based on the determined an- gular speeds and predefined threshold values for the angular speeds of the motors.
• PLL Phase Locked Loop
• Zero Cross Detector adapted to receive measurements of a variable of the variable power supplied to each of the motors of the subgroup, to determine the frequencies of the power supplied to the motors based on the measured variable, to calculate the angular speeds of the motors based on the determined frequencies, and to determine whether said predefined criteria are fulfilled or not based on the determined an- gular speeds and predefined threshold values for the angular speeds of the motors.
• Fig . 1 shows an apparatus for monitoring and limiting the velocity of an industrial robot according to a first embodiment of the invention .
• Fig . 2 shows a part of an apparatus for monitoring and limiting the velocity of an industrial robot according to a second embodiment of the invention .
• Fig . 3 shows an apparatus for monitoring and limiting the velocity of an industrial robot according to a third embodiment of the invention .
• An industrial robot includes a manipulator and a control system for automatically operating the manipulator.
• the manipulator includes one or more arms. Each arm includes a plurality of arm parts and a hand that are movable relative each other about a plurality of joints.
• One common type of industrial robot includes one arm having six joints.
• the manipulator includes a stationary foot, usually referred to as the base of the robot, which supports a first arm part, which is rotatable about a first joint.
• the first arm part supports a second arm part, which is rotatable about a second joint.
• the second arm part supports a third arm part which is rotatable about a third joint.
• the third arm supports a hand which is rotatable about a fourth , a fifth and a sixth joints.
• the robot hand comprises a tool attachment named a faceplate, which is adapted to hold a tool or a work object, in which an operating point, called TCP (Tool Centre Point), is defined .
• TCP Tool Centre Point
• the movements of the joints are driven by motors mounted close to the joints.
• the velocity of the tool centre point or the faceplate of the robot hand has to be monitored , and if the velocity exceeds a certain threshold value an action to limit the velocity of the robot is executed .
• the angular speed of some of the motors of the robot are supervised .
• the angular speed of a motor is measured as RPM (revolutions per minute).
• the velocity of the robot arm is commonly measured in m/s.
• a satisfactory result is achieved by supervising only three or four of the motors driving the joints depending on the length of the arm parts.
• it has to be determined which of the motors to be supervised . This is done by determining the influence of each mo- tor on the arm velocity, and selecting the motors which have the most dominating influence on the velocity of the robot arm . This is, for example, done by off-line simulations of the robot. For example, for a conventional robot having six joints, the motors of joints, 1 ,2, 3 and possibly the motor of joint 4 are selected to be supervised . Joints 5 and 6 have little influence on the velocity of the robot arm and accordingly the motors driving those joints are not selected .
• threshold values for a parameter to be super- vised have to be determined .
• the parameter must be related to the angular speed of the motors and the threshold values for the parameter are determined based on maximum allowed angular speed for the selected motors. Thus, maximum allowed angular speed for the selected motors have to be determined . This is, for example, done by off-line calculations and simulations of worse case robot velocities.
• one or more predefined criteria for allowed robot arm velocity are set up.
• the criteria are defined with respect to maximum allowed angular speed of the motors of said subgroup of motors.
• the criteria include the predefined threshold values for the parameter for the motors of the subgroup.
• the criteria may include threshold values for individual motors as well as for combinations of motors. In the following , a few examples of criteria for the robot mentioned above having six joints are listed :
• V1 ⁇ V T i and V2 ⁇ V T2 and V3 ⁇ V T3 and V4 ⁇ V T4 A second example of a criterion : V1 + V2 + V3 + V4 ⁇ V T
• V Ti is the threshold value for the angular speed of the motor i
• V Tij is a threshold value for the angular speed of a combination of motors.
• the criteria includes threshold values for the angular frequency of the selected mo- tors.
• a fourth example of criteria f1 ⁇ f T1 and f2 ⁇ f T2 and f1 + f2 + f3 + f4 ⁇ f T
• f is the frequency of the motor i
• f Ti is a threshold value for the frequency of the motor i
• f T is a threshold value for the frequency of a combination of motors. All criteria must be fulfilled to allow the robot to continue with the present arm velocity. In case any of the criteria is not fulfilled the arm velocity must be limited by reducing the speed of one or more of the motors of the robot.
• the speed of the tool center point, TCP can for example be calculated with help of Denavit-Hartenberg , D-H , transformation .
• the D-H transformation for a link i is
• A, Rot ZJ Trans z , di Trans x ,aiRot x ,ai
• the four quantities a, , a,, d, and are link length , link twist, link offset and joint angle and are parameters describing link i and joint i .
• the motors maximum allowed RPM is known in advance and can be used as a threshold level . If needed it is possible to add margins to compensate for the measurement inaccuracy. Knowing the maximum electrical angular speed , the minimum allowed time between two zero crossings of the sine wave can be calculated . This is an example when zero crossing of the motor phases is used to monitor the motors angular speed .
• poles is the number of poles of the motor.
• the frequency of the zero crossings for the sine wave is then proportional to the actual speed of the motor.
• the frequency to survey is calculated in advance i .e. offline and is used as threshold values to the hardware responsible for monitoring and limiting the speed . For example, by measuring the time between the zero crossing it is possible to detect if the motor runs faster than allowed and if such over speed is detected a speed limitations must be activated . This can , for example, be done with an ordinary frequency to voltage converter. It is even possible to add the individual voltages to have a more sophisticated surveillance with discrete analogue components. Another solution can be through programmable logic, or a combination thereof.
• Figure 1 shows an apparatus 1 for monitoring and limiting the velocity of an industrial robot according to a first embodiment of the invention .
• the robot has six joints and includes six motors M 1 -M6 for driving the six joints.
• Four motors M 1 -M4 has been selected and those are included in the subgroup of motors to be monitored .
• the measuring system includes one measuring unit 2 for each motor M 1 - M4, adapted to continuously measure the parameter of the motor.
• the measuring unit 2 can be any of a plurality of known types for measuring the motor frequency or speed , such as a camera, an accelerometer, a Hall component, an encoder, or a resolver.
• the motor frequency or motor speed can also be de- termined based on back EMF from a motor model , MRA ( Model Reference Adaptive), open loop models, observer models, exploited anisotropies, rotor slot harmonics, main inductance saturation , artificial saliency, or rotor slot leakage.
• the apparatus further includes a logic unit 3, adapted to receive values of the measured parameter from the measuring units 2 and to determine whether or not one or more predefined criteria for allowed robot arm velocity are fulfilled based on the measured parameter.
• the logic unit 3 may include discrete analogue components, such as filters, amplifiers, comparators, program- mable logic, as an FPGA, or a combination thereof.
• the apparatus also includes a data storage 4 , such as a Flash memory, RAM , ROM , DRAM , EEPROM etc, or as an analogue level for storing or setting the threshold values for the parameter.
• the threshold values are, for example, stored in a look up table.
• Output from the logic unit 3 is one or more signals for limiting the velocity of the robot in case any of the criteria is not fulfilled . Thus, it is possi ble to detect if one or more of the motor run faster than allowed and to activate velocity reduction if such over speed is detected .
• the reduction of the velocity can be done by disable the gate drives, which is safe torque off, by turning off the power to the DC-bus, disconnect the motor, or engaging the brakes, add a PWM pattern to the gate drives to continue at maximum allowed speed or to smoothly stop the motor, etc. I n all cases the motor speed will be limited before the motors contri bution to the velocity of the object exceeds requested limit.
• FIG. 2 shows a three phase motor M 1 driving one of the joints of the robot.
• a DC/AC converter 6 converts DC current to a vari- able alternating current in dependence on control signals from a control unit 7.
• the variable alternating current has three phases and is supplied to the motor M 1 .
• the speed of the motor depends on the frequency of the supplied alternating current.
• a measuring device in this example a resolver R, measures the angular position of the motor. The measurements are transferred to a servo unit 8, which uses the measurements to control the speed of the motor.
• Transducers 9 are arranged to measure the variable alternating current supplied to the motor M 1 . At least two of the phases are provided with a transducer 9. The transducers 9 are used for supervising the current supplied to the motor M 1 .
• a breaker 1 0 is provided for breaking the DC current supplied to the DC/AC converter 6.
• figure 2 shows a part 1 1 of an apparatus for monitoring and limiting the velocity of a robot according to an embodiment of the invention .
• the part 1 1 includes a determining unit 12 configured to receive current measurements from at least two of the transducers 9 and to determine a parameter, such as the angular speed or the frequency of the motor, to be used for supervising the motor. Current measurements from at least two of the phases are used to determine the parameter. This gives a redundancy and ensures that the surveillance and limitation will not fail if a single channel is lost.
• the determining unit 1 2 includes, for example, a PLL (Phase-Locked Loop) or a zero cross detector.
• determining unit includes hardware for determining the frequency of the motor based on the measured current and hardware for converting the determined frequency to the angular speed of the motor, to be used for supervising the motor.
• the part 1 1 includes a comparator 14 and a memory 1 6 for storing a threshold value for the parameter.
• the threshold values can be analogue voltage levels.
• the parameter is the angular speed in RPM of the motor and the criterion is that the measured speed of the motor V1 must be lower than a threshold value V T1 : V1 ⁇ V T1 If this criterion is not fulfilled , the comparator generates a signal to the breaker 10, which interrupts the DC- power supply to the DC/AC converter 1 .
• Figure 3 shows an apparatus for monitoring and limiting the velocity of an industrial robot including at least four joints, according to a third embodiment of the invention .
• the figure shows three motors M 1 -M3, which has been selected to be included in the subgroup of motors to be supervised .
• Maximum allowed an- gular speeds of the motors M 1 -M3 has been determined in advance, and is used as threshold values for the criteria. If needed it is possi ble to add margins to threshold values to compensate for the measurement inaccuracy. Knowing the maximum electri- cal angular speed , the time between two zero crossings of the sine wave of the variable power supply can be calculated :
• the frequency of the zero crossings for the sine wave is then proportional to the actual speed of the motor.
• the frequency to survey is calculated in advance i .e. offline and is used as threshold values to the hardware responsible for monitoring and reducing the velocity. By measuring the time between the zero crossing , it is possible to detect if the motors runs faster than allowed and if such over speed is detected a speed reduction is activated .
• the apparatus shown in figure 3 includes measuring units 20 configured to measure a variable, such as current or voltage of the variable alternating current supplied to the motors M 1 - M3 and to provide an output which is proportional to the frequency of the motor.
• the measuring unit includes, for example, a PLL or a zero cross detector.
• a zero cross detector measures the variable alternating current supplied to the motor and detects the zero crossings of the current.
• the PLL measures the frequency of the current.
• the apparatus also includes components 22 adapted to convert the output from the measuring units to the motors mechanical frequency.
• the components 22 are, for example a counter or a timer. Alternatively, the components 22 can be adapted to convert the output from the measuring units to the motor speed . I n that case, the threshold values are maximum allowed speeds for the motors.
• the criteria are f1 ⁇ f T and f1 + f2 + f3 ⁇ f Tsum
• the predetermined threshold values f T and fisumfor the frequencies are stored in data storage 24.
• the sum f1 + f2 + f3 of the frequencies is calculated in an itemizing totalizer 26.
• a first comparator 27 compares the measured frequency f1 with the threshold value ⁇ f T from the data storage 24, and if the criterion f1 ⁇ f T1 is not fulfilled , a signal is generated for limiting the velocity of the robot arm.
• a second comparator 28 compares the sum of the measured frequencies f1 + f2 + f3 with the threshold value f Tsum from the data storage 24, and if the criterion
• the present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims.
• the number of joint of the robot may vary and the number of motors supervised .

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• Engineering & Computer Science (AREA)
• Robotics (AREA)
• Mechanical Engineering (AREA)
• Human Computer Interaction (AREA)
• Manufacturing & Machinery (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Automation & Control Theory (AREA)
• Manipulator (AREA)
• Control Of Electric Motors In General (AREA)

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• 2010
  • 2010-10-28 WO PCT/EP2010/066389 patent/WO2012055440A1/en active Application Filing
  • 2010-10-28 EP EP10771131.9A patent/EP2632653B1/de active Active
  • 2010-10-28 CN CN201080069782.4A patent/CN103180106B/zh active Active

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